Waterborne Parasite Cerkarier Causes Itching Among Swedish Swimmers in Summer 2026

During the summer of 2026, swimmers in several Swedish bathing spots, including Långholmens badplats in Stockholm and areas around Gränna, Laholm, and Alvesta, have been affected by the waterborne parasite cerkarier. These larval flatworms, which normally inhabit aquatic birds and some freshwater snails, can attach to human skin during a certain lifecycle phase, causing intense itching and blisters. According to Stina Drakare, a researcher at the Swedish University of Agricultural Sciences (SLU), although cerkarier attach to humans, people are not suitable hosts, and the resulting skin irritation is harmless and typically resolves within a week.

Drakare advises swimmers to avoid water that appears green or murky and to choose swimming spots where municipal water quality tests are routinely conducted. Areas with many aquatic birds and snails pose a higher risk of cerkarier exposure. Swimmers are also encouraged to shower and change clothes immediately after swimming to minimize irritation. Despite improvements in water quality over the decades, previous challenges including algal blooms and wastewater discharge—especially in areas like Mälaren—have affected swimming conditions. Climate change threatens to worsen water quality by fostering algal growth under warm conditions, coinciding with peak swimming seasons.

Notably, cerkarier have existed in Sweden since at least Viking times, though their impact on bathers has become more noticeable in recent summers.